The F. R. Arcidiacono et al. U.S. Pat. No. 4,251,326 discloses thin film-type electrical substrate circuit devices in which thin film RC networks are fabricated on a substrate utilizing alpha tantalum capacitors, tantalum nitride resistors and electrical conductors, including associated electrical contact pads. Frequently, to reduce cost and to increase the circuit density of the devices, in another known process certain of the contact pads of the thin film RC networks are interconnected by thick-film crossunder conductors utilizing an arrangement in which thick film conductors are formed on the substrate prior to forming the thin film RC networks on the substrate.
More specifically, initially the substrate is cleaned in a known manner and the thick film conductors are formed on the substrate from a gold-filled paste material, such as by screening. After firing of the thick film conductors in a furnace, a coating of an electrically insulating glaze material is applied over an intermediate portion of each thick film conductor and opposite end portions of the thick film conductor are left unglazed. A second coating of the electrically insulating glaze material then is applied over the glazed intermediate portions of the thick film comductors. At the same time, a coating of the electrically insulating glaze material is applied to areas of the substrate in which capacitors are to be formed on the substrate. Following heat-curing of this glaze material, a thin protective layer of a film-forming metal, such as tantalum, is deposited on the substrate, including the unglazed opposite end portions and the glazed intermediate portions of the thick film conductors, by conventional sputtering techniques.
Next, portions of the tantalum layer above the unglazed end portions of the thick film conductors are etched away utilizing a photolithographic masking process, to form "windows" in the tantalum layer and thereby to expose the unglazed conductor end portions. The remainder of the tantalum layer then is thermally oxidized to produce a tantalum pentoxide underlayer which protects the substrate from attack by corrosive etchants during the course of subsequent processing. The thin film resistors, capacitors and conductors of the RC networks then are formed on the tantalum pentoxide layer, with associated contact pads of the networks being formed in direct contact with the exposed unglazed end portions of the thick film conductors, utilizing known material deposition, photolithographic masking, and etching techniques.
The foregoing procedure, in which "windows" are etched in the tantalum film layer above the unglazed opposite end portions of the thick film conductors, possesses a number of disadvantages. For example, the etching operation actually involves several steps, including the application of photoresist, exposure of the photoresist through a mask defining the "windows", removal of the unexposed photoresist, etching of the "windows" in the tantalum film layer, removal of the exposed photoresist and rinsing of the etched substrates in preparation for the next processing step. In addition, the forming of the "windows" in the tantalum film layer exposes the unglazed end portions of the thick film conductors to possible damage from etchant solutions used in the fabricating process, and/or exposes the end portions to contamination from the atmosphere, with deleterious effects upon the electrical characteristics of the subsequently formed thin film RC networks.
Accordingly, a purpose of this invention is to provide a simplified process of fabricating a film-type electrical conductor crossunder-crossover substrate circuit device which does not adversely affect the electrical characteristics of the device.